Transistors are widely used as electronic switches. Owing to the high switching frequencies with which transistors can be operated, transistors are not only suitable as “static” switches that are closed over a relatively long period of time, such as a few seconds, minutes or hours, but are also suitable for the clocked or pulsed control of loads.
Transistors controlled in a clocked or pulsed manner are used for example in driver circuits for inductive loads, such as, for example, in half-bridge or full-bridge drivers for electric motors, solenoid valves, etc. A further field of use is switching converters or switch-mode power supplies, in which a clocked control of a transistor serves for regulating the current consumption and thus for regulating output voltage.
In order to minimize conduction losses which can occur when the transistor is controlled in the on state, transistors used as switches are usually operated with a control voltage having a magnitude such that the transistor in normal operation is always operated in its linear region (ohmic region). In this context, “normal operation” means that there flows through the transistor a current which the transistor can carry permanently without the risk of destruction. However, if a short circuit occurs in the load controlled by the transistor, a high control voltage causes a very high current to flow through the transistor, which the transistor cannot permanently carry without the risk of destruction arising.
In order to detect such a short circuit it is known, in principle, to evaluate the current flowing through the transistor or a voltage drop across the load path of the transistor and to control the transistor in the off state, or to turn it off, if the presence of a short circuit is detected. On account of signal propagation times during the evaluation of the load path voltage or of the load current and on account of switching delays of the transistor, although the time duration during which a short-circuit current flows can be reduced, the high short-circuit current cannot be completely prevented. However, under certain circumstances, a short-circuit current that flows only for a very short duration already suffices to damage or even to destroy the transistor.